The present invention relates to a rack and pinion steering apparatus and a method for manufacturing a pinion, and is particularly directed to a method for forming a two-piece helical pinion for a rack and pinion steering apparatus.
A typical rack and pinion power steering apparatus for use in a power-assisted vehicle steering system includes a rack operatively coupled with steerable vehicle wheels and a pinion operatively coupled with a vehicle steering wheel. Teeth on the pinion are meshed with teeth on the rack such that rotation of the pinion produces linear movement of the rack which, in turn, causes the steerable wheels to turn laterally of the vehicle.
The teeth on the pinion can extend parallel to the central axis of the pinion, or can alternatively extend at an angle relative to the central axis in a pattern such as a helical pattern. It is desirable to have helical teeth on a pinion which extend at an angle of greater than 15xc2x0 because a higher angle accommodates a greater range of potential vehicle applications and creates a smoother feel to the vehicle driver when turning the vehicle steering wheel.
It is known to manufacture a pinion, including the forming of teeth in a helical pattern on the outer surface of the pinion, using a machining process. The machining process produces a relatively large quantity of waste material. It is also known to manufacture a pinion having helical teeth using cold forming processes. One known cold forming process begins with a single piece of a metal material which is first extruded to form some of the features of the pinion teeth, and which is subsequently placed into a hobbing machine to cut the helical teeth in the material into their final form. This known process is not capable of efficiently mass producing pinions with a helical tooth angle over 15xc2x0 because the large forces required to eject the helical pinions from the cold forming press destroys the tooling in the machine.
The present invention is a method for forming a helical pinion gear for a rack and pinion steering apparatus. The method comprises the steps of:
providing a cylindrical first blank made of a deformable material, the first blank having an outer surface and oppositely disposed first and second ends;
providing a cylindrical second blank made of a deformable material, the second blank having an outer surface and oppositely disposed first and second ends;
forming a bore extending at least partially through the second blank; forming helical teeth on the outer surface of the second blank; and interconnecting the first blank with the second blank to form the helical pinion.
The present invention also provides a rack and pinion steering apparatus for turning steering wheels of a vehicle upon rotation of a vehicle steering wheel. The rack and pinion steering apparatus comprises a housing having a chamber, and a rack linearly movable in opposite directions in the chamber to effect turning of the steerable vehicle wheels in opposite directions. The rack has an outer surface portion which includes rack teeth. A pinion is operatively coupled for rotation with the vehicle steering wheel. The pinion has an outer surface which includes pinion teeth extending in a helical pattern. The pinion teeth are meshed with the rack teeth to cause the rack to move linearly upon rotation of the pinion. The pinion comprises coaxially disposed first and second members fixedly attached to one another. Each of the first and second members has an inner surface and an outer surface. The inner surface of the second member engages the outer surface of the first member. The helical pinion teeth are formed on the second member.